Paper overlays, including paper overlays saturated with phenolic resin, have been used to improve the surface properties of wood-based substrates such as plywood, particleboard, waferboard and oriented strandboard (OSB). These overlays have been made with various basis-weight papers, resin contents, additives and other constituents in order to perform a variety of functions. For example, as defined in U.S. Product Standard PS.1-83 (American Plywood Association), the most common phenolic resin-saturated overlays used in North America are Medium Density Overlays (MDOs) and High Density Overlays (HDOs). Moreover, Phenolic Surface Films (PSFs) are overlays widely used throughout the rest of the world including a growing, but limited, market in North America.
MDOs have phenolic resin contents ranging from about 22-45% by weight, are generally prepared using heavier basis-weight papers, and contain nearly fully-cured (i.e., "C-staged") saturating resin. In contrast, HDOs and PSFs are generally prepared from lighter weight papers, have resin contents greater than about 45% by weight, and contain only partially-cured (i.e., "B" staged) saturating resin. In addition, currently-available MDOs are typically manufactured having a coating of thermosetting adhesive on one side of the overlays for adherence to wood-based substrates, whereas HDOs and PSFs do not include this adhesive-coating since they are self-bonding during subsequent hot-press application to wood-based substrates due to their high resin content.
MDOs, HDOs and PSFs are typically applied to one or both surfaces of a wood-based substrate by pressing in either a stepwise or continuous manner between the heated platens of a press. Flat or textured carrier cauls are commonly employed to impart the desired surface appearance to the overlay, and to prevent contamination of the platen surfaces. Typically, the formation of the wood-based substrate and application of the phenolic resin-saturated overlay take place concurrently in what is termed a "one-step" process. Alteratively, formation of the wood-based substrate and application of the phenolic resin-saturated overlay may involve separate pressing operations, in which case manufacture is termed a "two-step" process. Pressing conditions are dependent on the exact nature of the overlay and finished product being manufactured, but temperatures of 250.degree. F.-400.degree. F. and pressures of 200-350 pounds per square inch (psi) are typical. Duration of pressing depends on platen temperature and on heat transfer conditions in the press, and cycle times of 4-10 minutes are commonly employed.
MDOs typically are used to upgrade wood-based substrates by providing a smoother surface with better paintability, as well as improving resistance to mechanical abrasion and/or chemical attack. In addition, MDOs may be used as "cushion sheets" under HDOs and PSFs. The greater thickness, stiffness and tensile strength of MDO provide an effective cover for surfaces of wood-based substrates not suitable for direct application of HDO or PSF alone. Since the quality of available timber has decreased dramatically in many parts of the world due to depletion of old growth forests, the need for overlays to upgrade inferior wood surfaces, as well as cover wood products made from "young" growth resources, is expanding.
A major application area for overlaid wood-based substrates has been in the construction of forms for the casting of concrete. For this purpose, overlaid wood-based substrates have performed well, exhibiting a good combination of ease of machinability, durability and resistance to the aggressive chemical agents present in wet concrete, while also imparting the desired surface to the finished concrete. For example, U.S. Pat. No. 5,089,348, to Louderback (incorporated herein by reference in its entirety), discloses concrete forms made by adhering a pre-cured laminate overlay to a plywood substrate. The Louderback patent also provides an overview of existing technology used to manufacture concrete forms using plywood overlaid with MDO and/or HDO.
However, a major problem arises with the use of existing overlaid plywood concrete forms. In order to further waterproof the overlaid surface of the form in contact with the concrete (i.e., "the working surface" of the form), and to provide a shear plane to aid in the release of the form's working surface from the cured concrete, the working surface is coated before each use with a form release mixture which generally includes a hydrocarbon oil, waxy substances and/or other chemical agents in a hydrocarbon solvent. These form-release mixtures (referred to as "form oils") ease release of the cured concrete from the working surface of the form by formation of a physical discontinuity (i.e., an oil-water interface) between the form's working surface and the cured concrete. Moreover, chemical additives (e.g., fatty acids and fatty acid esters) may be added to form oils to generate a "reactive release" surface. Such chemical additives act by forming a weak, chemically-discontinuous phase on the working surface of the form which provides additional assistance in the subsequent release of the cured concrete.
With increased environmental pressure to limit the escape of volatile elements into the atmosphere, and avoid possible contamination of soil and ground water from construction site runoff or spillage, coating of concrete forms with form oils have come under increased scrutiny. Depending upon the amount of mechanical fitting (e.g., installation of reinforcing iron, electrical conduit, utility pipes, etc.) which must be completed before the concrete is poured, the form oils may be exposed to the elements and worker traffic for an extended period. In hot areas of the world, for example, the surface temperatures of horizontal concrete forms can easily exceed 150.degree. F. Such temperatures can volatilize various components of the form oils, thereby reducing its effectiveness and contributing to environmental pollution. Rain exposure and attendant runoff can likewise lead to environmental contamination, while also diminishing performance of the form oils.
Accordingly, there is a need in the art for concrete forms which do not require application of form oils to the working surfaces thereof, or which can greatly reduce the amount necessary to impart acceptable release properties to the form. There is also a need for overlays that can effectively cover poorer quality solid substrates (particularly wood-based substrates) and produce a surface comparable or superior to that now obtained by combining one or more sheets of HDO or PSF with an underlayment of MDO. In addition, there is a need in the art for methods of manufacturing such improved overlays, as well as solid substrates having such overlays adhered to a surface thereof. The present invention fulfills these needs, and provides further related advantages.